The present invention relates to the treatment of liquids, and, in particular, to a device for coalescing a liquid out of a liquid-liquid suspension, for vacuum dehydration, and for thin-film evaporation.
Several different processes have been used for recycling oils to remove water and other contaminants from the oil. Among the processes are thin film and wiped film vacuum distillation, vacuum dehydration, filtration, clay treating, centrifuging, coalescing, and decanting.
The first step in the recycling of any industrial lubricant is to remove the free water. This can be accomplished by heating the oil up and allowing the free water to settle. However, it takes a substantial amount of time for settling to occur and requires substantial storage capacity. In the case of portable recycling units, for which the present invention is intended, the time limitation is important and there is very limited storage, so settling tanks are not an option for portable units.
Many recycling businesses use centrifuges to spin out the free water. These are somewhat effective, but they cannot remove all the solids and free water, they require a great deal of maintenance, and their efficiency can be greatly reduced by changes in feed conditions.
Many recycling businesses use cartridge coalescers to remove free water from the oil, but these are not very efficient, require storage for settling, and require several passes through the cartridge to remove any substantial amount of water, thereby making the process slow.
Once most of the free water has been removed, the rest of the water must be removed. This can be done in several ways. First, the oil can be heated to 250 degrees F. to drive off all the water. This is a slow, energy intensive method, which also tends to oxidize the oil and destroy additives in the oil. It is possible to remove the water chemically, but this also tends to destroy the additives and oxidize the base oil.
Another method for removing the water is vacuum dehydration. The known devices for using this technique use diffuser cartridges in a vacuum chamber. The diffuser cartridges have a very limited surface area, tend to plug frequently, and have very low production rates if the oil has more than one percent water, which is usually the case in recycling. Another problem with vacuum dehydration is the generation of what is known as "rag". This is considered an unrecoverable material (which looks like a shredded rag) that is created when oil and water are mixed together in certain circumstances.
Another method for purifying the oil is through thin-film distillation or evaporation. Most thin-film evaporators have movable parts (such as scrapers) to keep the thin film in contact with the surface of the heat exchanger. These movable parts must be manufactured with very close tolerances and are expensive and require maintenance. Other types of evaporators tend to plug and to foam.